This invention relates to the field of actuators, specifically combustion-powered actuators tolerant of misfires and suitable for use with minimal electrical energy input.
Electrical power is commonly used for actuation. A wide variety of electrical motors, solenoids, and other actuators are known to those skilled in the art. Electrically-powered actuation is generally considered efficient, but is dependent on access to electrical generation or storage facilities. Access to electrical generation or storage facilities can be problematic in some applications. For example, mobile systems such as mobile robots often can not be tethered to a electrical power supply and must rely on battery storage or on-board generation. On-board generation can be impractical in many environments, and the mass of battery storage can dramatically reduce the performance of the mobile system.
Combustion power is also widely used for actuation. Combustion power requires access to appropriate fuel. Combustible fuel can be significantly more efficient energy storage than batteries. Accordingly, applications that require mobility often rely on combustion-powered actuation. For example, combustion power is used in most automobiles and similar vehicles.
Conventional combustion-powered actuation relies on substantially continuous operation. A conventional internal combustion engine uses significant battery power to spin the engine until reliable combustion operation is underway, then uses the inertia of the engine to overcome misfires or other operational irregularities. Some applications, however, require intermittent operation either because of the nature of their operation (for example, impulse operation such as needed for a hopping robot) or because of the nature of the application (for example, operation only in response to certain stimuli such as in remote sensing and control applications). Such applications are not suited for conventional internal combustion actuation. A combustion-powered actuator suitable for intermittent operation would benefit such applications, but poses significant complications relating to carburetion, fuel metering, ignition, and exhaust gas purging, relative to conventional internal combustion engines.
For intermittent operation, the actuator should have the ability to operate without external intervention. This is termed cold start capability. For conventional internal combustion engines a starting system consisting of a starter motor and battery usually provides this function. During short intervals where power is not required from the engine it simply idles: runs at low speed consuming little fuel and doing no useful work. In the case of an intermittent actuator, however, there is no state comparable to idling of an internal combustion engine, so every actuation can be viewed as a cold start. The use of significant electrical energy to provide a cold start capability can require significant battery resources, detracting from the advantages of combustion-powered actuation.
Closely related to cold start and potentially more limiting is misfire tolerance. A misfire is a condition where the fuel-air mixture fails to ignite when the ignition system fires. After a misfire, the combustion chamber must be purged to remove the fuel-air mixture, new fuel and air must be introduced, and the ignition system must fire again. If a conventional internal combustion engine misfires, the engine can coast through the misfire and onto the next power stroke performing all the necessary functions to tolerate the misfire. An efficient intermittent actuator need not have any continuously moving mechanical parts, so a misfire must be tolerated by using other forms of energy. If misfires are significantly less frequent than cold starts, the expenditure of small amounts electrical energy can be acceptable. However, it is preferable if the system does not require additional energy to tolerate a misfire.
A third major challenge is atmospheric pressure carburetion. Carburetion consists of combining fuel and air and introducing them into the combustion chamber. Introducing fuel into the chamber is relatively straightforward since fuel volume is small compared to the combustion chamber volume and the fuel system can easily be pressurized. Introducing air into the combustion chamber is another matter. Conventional four-stroke internal combustion engines draw air into the cylinder by means of the vacuum generated during the intake stroke. Conventional two-stroke internal combustion engines draw air into the crankcase under vacuum and then discharge it to the cylinder under pressure. Open flame combustion devices such as propane torches and pressure lanterns use an accelerated fuel stream to produce a Bernoulli effect to entrain the required air. If an intermittent actuator is normally in a cold start mode and must be able to actuate after extended dormant periods, then maintaining the combustion chamber under vacuum can be problematic. The use of an entrainment system also presents problems because the fuel-air mixture must be introduced into a closed combustion volume. Entrainment carburetion only works for an open flame where the downstream pressure is never above atmospheric.
In addition to the above challenges, igniting the fuel-air mixture can be considerably more difficult than in a conventional internal combustion engine. First, if there is no compression stroke then uncompressed fuel-air mixture must be ignited. Conventional internal combustion engines typically use compression ratios of 8:1. This means that the volumetric energy density of the fuel-air mixture in an atmospheric pressure combustion chamber is only 1/8 as great as that of a conventional internal combustion engine. Also, in a conventional internal combustion engine the adiabatic compression of the fuel-air mixture raises the temperature by about 400 C. The combination of lower energy density and lower temperature in an atmospheric pressure combustion-powered actuator can make ignition much more difficult. One difficulty is that the combustion chamber for an intermittent actuator must be more completely purged than for an internal combustion engine. Conventional four-stroke engines leave about 15% of the volume unpurged. Conventional two-stroke engines leave about a 40% unpurged. Experiments with atmospheric pressure combustion show that less than 5% of the combustion chamber volume can be left unpurged for ignition to be practically achieved.
Accordingly, there is a need for improvements in internal combustion technology that allow intermittent combustion-powered actuation.